The invention relates to a process for operating a system for the water vapor reforming of methanol. Mobile systems for water vapor reforming of methanol of the generic type are operated, for example, in fuel-cell-operated motor vehicles in order to obtain hydrogen for the feeding of the fuel cells. For operating such systems, it is known to vary the amount of the processed water vapor/methanol mixture which is introduced into the reforming reactor as a function of the load condition of the system. In German Patent Documents DE 1 949 184 and DE 21 57 722 C2 (in which methane is used instead of methanol), for example, a jet pump is provided for this purpose, and the vapor flow is adjusted as a function of the loading condition of the system. Thus, during a load change, the methane inflow also changes corresponding to the jet pump characteristics.
U.S. Pat. No. 5,401,589 discloses a process for operating a fuel cell system in a motor vehicle by means of a reformer system which is connected in front of the fuel cells and uses water vapor reforming of methanol for the purpose of obtaining the hydrogen required for the fuel cells. In order to be able to react to load changes faster than by merely changing the quantity of the water vapor/methanol mixture introduced into the reforming reactor, a buffer battery and an oxygen reservoir are provided. In the case of an increased load requirement, additional oxygen is taken from the oxygen reservoir for introduction into the fuel cell system. It is also known from this as well as other documents that, when the water vapor concentration in the water vapor/methanol gas mixture is too low, during its reforming an increased CO-concentration occurs in the reformate. This is undesirable, particularly when the reformate is used as a fuel in so-called PEM fuel cells because these are poisoned by carbon monoxide. Normally, the water vapor/methanol mixing ratio is therefore maintained within a range of between one and ten.
Japanese Patent Document JP 62-46902 (A) discloses an evaporator for a methanol reforming system which contains a coil-shaped evaporator tube whose diameter increases in steps from an upper area, which is adjacent to a burner, to a lower area, to take into account the increase in volume of the medium guided through the evaporator during the heating-up of the burner. This arrangement prevents an increase in the flow speed of the medium through the evaporator tube as the heating increases, so that the water/methanol mixture guided through the evaporator retains the capacity to absorb a sufficient amount of heat, thereby preventing undesirable reliquefying effects which would otherwise occur, particularly in the case of high supply quantities in the output-side lower portion of the evaporator tube, because of an excessive flow rate.
During the operation of systems for the water vapor reforming of methanol, it is observed that in the case of a load change (that is, a change of the quantity of water vapor/methanol mixture introduced into the reforming reactor), a change of the water vapor/methanol mixture ratio occurs, due to a momentary change of the evaporation conditions in the system. This, in turn, leads to fluctuations in the CO-concentration of the reformate, which can cause undesirably high CO-concentrations at times, which must be removed in an oxidizer which follows.
The object of the invention is to provide a process of the initially mentioned type which avoids the formation of undesirably high CO-concentrations in the reformate, particularly in the case of fast load changes.
This and other objects and advantages are achieved by the process according to the invention, in which the water vapor/methanol mixing ratio of the water vapor/methanol mixture introduced into the reforming reactor is maintained at a predeterminable desired value even during load changing operations, including sudden load jumps. As a result, during the operation of the system for the water vapor reforming of methanol, no undesirably high CO-concentrations occur in the reformate; that is, neither in the static nor in the dynamic load operation. The momentary water vapor/methanol mixing ratio can be detected in a conventional manner, either directly by means of a sensor system arranged in front of the reforming reactor input, or indirectly by measuring the CO-concentration in the reformate. Corresponding measures in the mixture preparation step ensure that the predetermined desired value of the mixing ratio will then be maintained. In particular, according to the process, excessive lowering of the water vapor/methanol mixing ratio below the predeterminable desired value, which is typically between one and three (and thus an excessive CO-concentration in the reformate) can be avoided.
In one embodiment of the invention, the predetermined desired value of the mixing ratio is maintained by injecting water and methanol by way of a valve, in a timed manner during the preparation step. The injection start and/or the injection duration of the valves can be appropriately changed as a function of the deviation of the actual value of the water vapor/methanol mixing ratio from the desired value.
In another embodiment, the predeterminable desired value of the mixing ratio is maintained during sudden load jumps by storing the prepared water vapor/methanol mixture in a pressure vessel at a constant pressure and a constant temperature. From there it is supplied at a constant water vapor/methanol mixing ratio to a superheater connected in front of the reforming reactor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.